T-Mobile got aggressive in 2013, positioning itself as the "UnCarrier" and actively courting customers of other mobile providers to switch.

According to the company, it's working -- at least in terms of adding new customers.

The fourth-largest mobile carrier in the United States was also the fastest-growing in 2013, adding 4.4 million customers, T-Mobile said Tuesday during a quarterly call with investors.

"What a year it's been," CEO John Legere said. "We've been extremely busy."

More than 1.6 million of those new customers arrived in the last three months of the 2013 fiscal year, marking the third quarter in a row that T-Mobile added more than 1 million paying users.

But it was also the third quarter in a row that the company lost money: $20 million, this time. That reflects the money T-Mobile is spending to attract new users and expand its network, as well as some of the less-expensive plans it's offering to lure all those new customers.

Not surprisingly, Legere and other T-Mobile brass weren't emphasizing that part on Tuesday. Instead, they cited the growth numbers again and again. And they do mark a turnaround from the 256,000 customers T-Mobile lost in 2012.

Under Legere's brash leadership, the company has taken some unconventional tacks to lure business.

In January, T-Mobile offered to pay up to $650 towards customers' early termination fees with other carriers if they switch. (Of course, that was a response to AT&T targeting T-Mobile customers with a similar deal).

And it didn't hurt that, last March, T-Mobile became an approved carrier for the iPhone for the first time.

"We changed the way this industry operates and customers responded," Legere said.

While acknowledging that the highest number of customer defections came from AT&T, the No.1 carrier in the United States, T-Mobile declined to give more detailed figures.

"We're not trying to be too cute about where the customers are coming from," Legere said. "That's not something we're going to get into anymore. As (mobile carriers) are all getting highly competitive, we're not going to give a further road map of where they're coming from."

T-Mobile also expanded its 4G LTE network in 2013. It now covers 209 million people and is in 95 of the top 100 metro areas in the U.S. The company has plans to further expand in 2014, eventually covering more than 250 million potential customers.